I have read the threads and can't find a similar problem poted.

The symptoms:

1. 91 350SDL with 132000.
2. Temp slowly rises to 100-105C and remains steady there.
3. New radiator
4. Replaced thermostat (twice)
5. Replaced explansion tank cap
6. Water pump new
7. Fluid level good with no loss

Any ideas on what to do next would be appreciated.

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Dick87

Check the temp. sensor ground wire, and the connections on your instrument cluster. If either of those two are loose or not in good shape, it could give you a false reading.

Other than that.. I wouldn't know. Sorry.

fan clutch?

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'85 300D Turbo - CA Version

Good point. How could I have missed that?

It all depends on the scenario, really. Is the car running hot on the highway, city, or idle? If it runs hot at idle, as he said.. your fan clutch is probably shot.

On the highway, however.. there should be enough ram-air to cool the radiator, even with a faulty fan clutch.

Fan clutch is working. Good tip on the temperature sensor. Next thing to try. PITA to get at as it is behind the IP. Temp seems to get hotter as I reach highway speeds. I agree that the air forced through the radiator at moderate highway speeds should help in reducing the temp which it does not.

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Dick87

You've done everything necessary to achieve proper temperature.

The fan clutch won't solve a problem of 105°C. It's just getting fully coupled at that temp. M/B thinks that 105°C. is perfectly acceptable.

So, my suggestions are as follows:

1) Borrow an infrared temp sensor and confirm the gauge is accurate. I'm betting that it's fine, but, you never know.

2) Pull the radiator and thorougly clean the condenser if this has never been done. It's surprising how much debris can accumulate in the unit and restrict the airflow. You need to use a compressor, a pressure washer, some condenser cleaner, or a combination of all three for a successful result. You should be working from the rear and forcing water/air/solution forward toward the front. You can't do this with the radiator in the vehicle.

Yep clean the condensor. Last year when I replaced mine I was amazed at the amount of crap built up in the old one. Lots of air simply wasn't making its way to the radiator.

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2006 CL500
1969 280SE
2023 Ram 1500
2026 Genesis GV70
2007 Tiara 3200

Thanks for the tips. I will have to locate an infrared sensor. Cleaning the condensor is next on the to do list.

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Dick87

If you have any bent fins on the condensor you can straighten them with a fin straightener
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=37892

Chapter 2. Cleaned the condensor coils as recommended. Cooling system performance improved as follows:

1. Moderate outside temperature - less than 75 F under moderate speed and load temp is less than 90C.
2. Run AC temp still less than 90C
3. Under load - up hill or higher speeds (greater than 75) temp approaches 100C
4. Higher outside temp - greater than 80F temp approaches 100C wihout AC - with AC 105C

Any ideas on how to check that I have good cooling flow? The radiator is hot all over the surface which I think means the cooling fluid is circulating through the whole radiator. My thought is maybe the water pump is not performing up to snuff.

Any other ideas?

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Dick87

Just for the heck of it check the top radiator hose after the car has cooled down for say 5 hours before starting it up. Is it slack or hard? I do not expect it to be hard but it never hurts to eliminate it. Easy and cheap test.

Upper radiator hose is soft (not pressurized).

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Dick87

Looks like it is time for some theories.

Brian Carlton - the schroud for the two cooling fans blocks 20-30% of the surface of the condensor. If after cleaning there is still some blockage is it reasonable to conclude that the cooling system is operating at the limits of its capacity and the shroud blockage pushes the system over its limit to cool under high cooling demand situations? The single fan on the 300sdl does not have a shroud and therefore very little condensor blockage.

Any thoughts on carving up the shroud to expose more condensor surface to air flow?

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Dick87

the shroud is there to help keep the air from spilling out the sides when the fans come on (when it is hottest). unless your fans arent working, cutting holes in the shrouds would only make it worse.

imho

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I agree, the shroud is there for a reason.

System burped properly? Expansion tank cap?

High speed driving (high boost/fuel use), A/C on, as well as high ambient... I'd expect the car to run in the 90°C range easily...

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]